1. Field of the Invention
The present invention relates to an electrophotographic photoconductor comprising a photoconductive layer which contains a bisazo compound. The present invention also relates to above-mentioned bisazo compound which is useful as an organic photoconductive material in the photoconductor and a method of preparing the bisazo compound, and in addition, relates to raw materials for the above-mentioned bisazo compound and the respective preparation methods of those raw materials.
2. Discussion of Background
Conventionally, the photoconductive material for use in the electrophotographic process is roughly divided into two groups, that is, an inorganic photoconductive material and an organic photoconductive material. The above-mentioned electrophotographic process is one of the image forming processes, through which the surface of the photoconductor is charged uniformly in the dark to a predetermined polarity, for instance, by corona charge. The uniformly charged photoconductor is exposed to a light image to selectively dissipate the electric charge of the exposed area, so that a latent electrostatic image is formed on the photoconductor. The thus formed latent electrostatic image is developed into a visible image by use of a toner comprising a coloring agent such as a dye or pigment, and a polymeric material. Such an electrophotographic process is called xe2x80x9cCarlson processxe2x80x9d.
The photoconductor employing the organic photoconductive material is advantageous over that employing the inorganic photoconductive material in terms of the degree of freedom in the wave range of the light to be employed, and the film-forming properties, flexibility, transparency, productivity, toxicity, and manufacturing cost of the photoconductor. In light of the above-mentioned advantages, most of the current photoconductors employ the organic photoconductive material.
Such a photoconductor is repeatedly operated in the copying apparatus employing the above-mentioned electrophotographic process or the like, so that the photoconductor is required to have excellent electrostatic properties, with respect to the photosensitivity, acceptance potential, retentivity of charge, potential stability, residual potential, and spectral sensitivity.
In recent years, the development of data processing apparatus employing the above-mentioned electrophotographic photoconductor is remarkable. In particular, there is a remarkable improvement in the printing quality and the reliability of the digital printer which is capable of converting data into digital signals and recording the data using a light. Such a digital recording system is applied not only to the printer, but also to the copying machine. Thus, the digital copying machine is actively developed. It is supposed that the demand for the digital copying machine will further increase in line with the addition of various data processing functions.
The photoconductor designed for the above-mentioned digital recording system is required to have special characteristics different from those required for the conventional analogue recording system. For instance, a semiconductor laser beam (LD) or a light emitting diode (LED) is widely employed as a light source for the digital recording system because of its compactness, cheapness and high reliability. The wave range of the currently used LD is within the near infrared region, and the wavelength of the currently used LED is 650 nm or more. Therefore, the electrophotographic photoconductors for use with the above-mentioned digital recording system are required to show sufficient sensitivity in the wavelength range from the visible region to the near infrared region.
A specific azo compound is conventionally known as a useful organic photoconductive material, in particular, a charge generation material, in the layered photoconductor. In the above-mentioned layered photoconductor, a charge generation layer and a charge transport layer are successively overlaid on an electroconductive support. The charge generation layer comprises a charge generation material capable of generating a charge carrier when exposed to light, and the charge transport layer comprises a charge transport material serving to efficiently inject the charge carrier generated in the charge generation layer into the charge transport layer and transport the charge carrier.
A variety of azo compounds for use in the photoconductor are conventionally proposed, for example, benzidine bisazo compounds in Japanese Laid-Open Patent Applications 47-37543 and 52-55643, stilbene bisazo compounds in Japanese Laid-Open Patent Application 52-8832, diphenyl hexatriene bisazo compounds in Japanese Laid-Open Patent Application 58-222152, and diphenyl butadiene bisazo compounds in Japanese Laid-Open Patent Application 58-222153.
However, the sensitivity of the electrophotographic photoconductor is slightly decreased when the above-mentioned conventional azo compounds are employed. Therefore, such a photoconductor is not suitable for the high-speed copying machine. Further, the sensitivity of the photoconductor is extremely low in the wavelength range of the LD, so that the photoconductor employing the conventional azo compound cannot be put to practical use in the field of digital recording system. There is an increasing demand for the preparation of an azo compound for use in the electrophotographic photoconductor, that is useful as an organic photoconductive material free of the above-mentioned conventional shortcomings.
There is reported 1,4-bis[4-(3-nitrophenyl)-1,3-butadienyl]benzene in J. Org. Chem. vol. 24, 1969 (1959) by R. N. McDonald and T. W. Campbell. A bisazo compound can be prepared from the above-mentioned 1,4-bis[4-(3-nitrophenyl)-1,3-butadienyl]benzene. However, the bisazo compound thus obtained does not show any absorption in the wavelength range of the semiconductor laser. This is because each azo group is bonded to phenyl group at the m-position in the large-size conjugated system of 1,4-bis[4-phenyl-1,3-butadienyl]benzene, so that m-phenylene bonds insulate the conjugation in the molecule. This mechanism is detailed in Nippon Kagaku Kaishi 1986, (3), P.379-386. Therefore, such a bisazo compound is not useful as the organic photoconductive material for use in the photoconductor that is required to show high sensitivity with respect to the wavelength of the currently used semiconductor laser beam.
Accordingly, a first object of the present invention is to provide an electrophotographic photoconductor capable of showing flat and high sensitivity over a wide wavelength range from the visible light range to the near infrared range so as to cope with a digital copying machine and a digital printer.
A second object of the present invention is to provide a bisazo compound useful as an organic photoconductive material in the electrophotographic photoconductor which has high sensitivity and can be put to practical use in the laser printer as well as the high-speed copying machine.
A third object of the present invention is to provide a method of preparing the above-mentioned bisazo compound.
A fourth object of the present invention is to provide raw materials for the preparation of the above-mentioned bisazo compound.
A fifth object of the present invention is to provide a method of preparing each of the above-mentioned raw materials.
The first object of the present invention can be achieved by an electrophotographic photoconductor comprising an electroconductive support, and a photoconductive layer formed thereon which comprises as the effective component at least one bisazo compound of formula (I): 
wherein Cp1 and Cp2 are each a coupler radical which may be the same or different.
The-second object of the present invention can be achieved by a bisazo compound of formula (II): 
wherein Z is a benzene ring, a naphthalene ring or a carbazole ring, each of which may have a substituent; R is independently a hydrogen atom, a halogen atom, a lower alkyl group, a lower alkoxyl group or nitro group; n is an integer of 1, 2 or 3.
The third object of the present invention can be achieved by a method of producing the bisazo compound of formula (II) comprising the step of allowing a bis(diazonium salt) compound of formula (III) to react with a coupler of formula (IV): 
wherein X is an anionic functional group; and 
wherein Z is a benzene ring, a naphthalene ring or a carbazole ring, each of which may have a substituent; R is independently a hydrogen atom, a halogen atom, a lower alkyl group, a lower alkoxyl group or nitro group; and n is an integer of 1, 2 or 3.
The fourth object of the present invention can be achieved by (i) 1,4-bis[4-(4-nitrophenyl)-1,3-butadienyl]benzene represented by formula (V): 
(ii) 1,4-bis[4-(4-aminophenyl)-1,3-butadienyl]benzene represented by formula (VI): 
(iii) a bis(diazonium salt) compound represented by formula (III): 
wherein X is an anionic functional group.
The fifth object of the present invention can be achieved by a method of producing 1,4-bis[4-(4-nitrophenyl)-1,3-butadienyl]benzene of formula (V) comprising the step of allowing a bis(phosphonium salt) compound of formula (VII) to react with 4-nitrocinnamaldehyde: 
wherein R is phenyl group or an alkyl group; and Yxe2x8ax96 is a halogen ion.
The fifth object of the present invention can also be achieved by a method of producing 1,4-bis[4-(4-nitrophenyl)-1,3-butadienyl]benzene of formula (V) comprising the step of allowing a phosphonate of formula (VIII) to react with terephthalaldehyde: 
wherein Z is a lower alkyl group.
Further, the fifth object of the present invention can be achieved by a method of producing 1,4-bis[4-(4-aminophenyl)-1,3-butadienyl]benzene of formula (VI) comprising the step of reducing 1,4-bis[4-(4-nitrophenyl)-1,3-butadienyl]benzene of formula (V).
Furthermore, the fifth object of the present invention can be achieved by a method of producing a bis(diazonium salt) compound of formula (III) comprising the step of subjecting 1,4-bis[4-(4-aminophenyl)-1,3-butadienyl]benzene of formula (VI) to diazotization.